Cosmic rays are mostly charged particles that reach Earth from outer space. Although they were discovered more than a century ago, the origin of the ultra-high energy ones (with an energy above 1018 eV, or 100,000 times more energetic than the particles in the LHC beam, the most powerful particle accelerator on Earth) is still a puzzle. The large area of the Pierre Auger Observatory of about 3000 km2 compensates for the low flux of such particles and allows us to search for an excess/deficit of events arriving from different directions in the celestial sphere.

Using events collected with the surface detector of the Pierre Auger Observatory, we searched for deviations from isotropy (same flux of events from any region of the sky) using two complementary analyses widely used in the literature: the angular power spectrum and the so called needlet wavelet analysis. The goal of these techniques is to search for regions in the sky with cosmic-ray overdensities at different angular scales, therefore, performing a multi-resolution study.The analyses were performed for different energy ranges: for events with an energy between 4 EeV and 8 EeV and for events with an energy above 8 EeV, where 1 EeV = 1018 eV. No deviation from isotropy is observed at any angular scale in the energy range between 4 and 8 EeV.The angular power spectrum of events with an energy above 8 EeV is shown in the left panel of Figure 1. Each angular moment ℓ corresponds to an angular scale of 180o/ℓ. The gray band region stands for the 99% confidence-level lower and upper bounds that would result from statistical fluctuations assuming an isotropic distribution. One can see that an indication of a significant dipole moment is captured, while no additional deviations from isotropy are observed for moments beyond the dipole one at ℓ = 1. The probability that this result arises from an isotropic distribution is 1.3 × 10−5 in the case of the angular power spectrum and 2.5 × 10−3 in the case of the needlet analysis. A sky map of the filtered needlet scale j = 0 (most sensitive to the dipolar scale) is shown in equatorial coordinates in the right panel of Figure 1 for the same energy bin. The excess of events points to (α, δ) = (97° ± 16°, −39° ± 17°) (with α the right ascension and δ the declination), with a deficit of events in the opposite direction.

Figure 1: Left: angular power spectrum for events with E ≥ 8 EeV: a clear indication for a dipole moment (ℓ = 1) is captured while no other deviation from isotropy is observed for moments beyond the dipole one. Right: sky map, in equatorial coordinates, for events in the same energy bin filtered with the needlet scale j = 0.

Future work will profit from increased statistics, and together with updated descriptions of the Galactic magnetic field will help in probing both the source distribution and the propagation regime of extragalactic cosmic rays needed to produce a density of particles outside of the Galaxy compatible with the one inferred from the kind of measurements performed in this article.